如何在SQLite中跨表对列添加唯一约束

Question

假设我有两列：一张表中的

GroupName

，另一张表中的

Username

。如何限制这些列的值，以便添加到一列的任何值在两列中都是唯一的？最好以“不混乱”的方式。

Answer 1

您提到的结构没有简单的方法。当然，有有实现触发器的方法，但是比较麻烦。

替代数据模型解决了这个问题：

create table things (
    thingsId int auto_increment primary key,
    name text,
    type text,
    . . .
    check (type in ('group', 'user')),
    unique (name)
);

这会将每种类型的事物放入单个列中，然后施加唯一约束。

Answer 2

您还可以使用触发器和

SELECT RAISE(FAIL, ...)

来执行此操作：

例如，假设我有一个

Shelf

表，我希望它包含

Book

和

Magazine

。我也不希望

Book

和

Magazine

在书架上有相同的标题。我真的很想对

(Shelf.ShelfID, Book.BookID OR Magazine.MagazineID)

有一个独特的约束，但该语言没有提供这一点。让我们：

创建表（注意：在现实世界的实现中，文档建议在外键上建立索引以提高性能）：

CREATE TABLE Shelf (
    ShelfID INTEGER PRIMARY KEY,
    Title TEXT NOT NULL,
    UNIQUE(Title)
) STRICT;

CREATE TABLE Book (
    BookID INTEGER PRIMARY KEY,
    ShelfID INTEGER NOT NULL,
    Title TEXT NOT NULL,
    FOREIGN KEY (ShelfID) REFERENCES Shelf(ShelfID) ON DELETE CASCADE,
    UNIQUE(Title)
) STRICT;

CREATE TABLE Magazine (
    MagazineID INTEGER PRIMARY KEY,
    ShelfID INTEGER NOT NULL,
    Title TEXT NOT NULL,
    FOREIGN KEY (ShelfID) REFERENCES Shelf(ShelfID) ON DELETE CASCADE,
    UNIQUE(Title)
) STRICT;

创建视图和触发器（这是神奇的部分）：

CREATE VIEW vw_Book_Magazine_Shelf_AllTitles AS
SELECT s.ShelfID, b.Title
FROM Shelf s JOIN Book b ON s.ShelfID = b.ShelfID
UNION ALL
SELECT s.ShelfID, m.Title
FROM Shelf s JOIN Magazine m ON s.ShelfID = m.ShelfID
;

CREATE TRIGGER tr_Book_Insert_CheckUniqueTitle
BEFORE INSERT ON Book
FOR EACH ROW
BEGIN
    SELECT RAISE(FAIL, 'title already exists in Shelf')
    FROM vw_Book_Magazine_Shelf_AllTitles
    WHERE ShelfID = NEW.ShelfID AND Title = NEW.Title;
END;


CREATE TRIGGER tr_Book_Update_CheckUniqueTitle
BEFORE UPDATE ON Book
FOR EACH ROW
BEGIN
    SELECT RAISE(FAIL, 'title already exists in Shelf')
    FROM vw_Book_Magazine_Shelf_AllTitles
    WHERE ShelfID = NEW.ShelfID AND Title = NEW.Title;
END;

CREATE TRIGGER tr_Magazine_Insert_CheckUniqueTitle
BEFORE INSERT ON Magazine
FOR EACH ROW
BEGIN
    SELECT RAISE(FAIL, 'title already exists in Shelf')
    FROM vw_Book_Magazine_Shelf_AllTitles
    WHERE ShelfID = NEW.ShelfID AND Title = NEW.Title;
END;


CREATE TRIGGER tr_Magazine_Update_CheckUniqueTitle
BEFORE UPDATE ON Magazine
FOR EACH ROW
BEGIN
    SELECT RAISE(FAIL, 'title already exists in Shelf')
    FROM vw_Book_Magazine_Shelf_AllTitles
    WHERE ShelfID = NEW.ShelfID AND Title = NEW.Title;
END;

最后，让我们测试一下：

INSERT INTO Shelf(ShelfID, Title) Values(1, 'Shelf Title');
INSERT INTO Book(BookID, ShelfID, Title) VALUES (1, 1, 'Book Title');
INSERT INTO Magazine(MagazineID, ShelfID, Title) VALUES (1, 1, 'Mag Title');
-- this fails
UPDATE Magazine SET Title = 'Book Title' WHERE Title = 'Mag Title';